Typical liquid crystal display cells heretofore developed include a field effect mode cell proposed by M. Schadt et al., Applied Physics Letters, Vol. 18, 127-128 (1971), a dynamic scattering mode cell proposed by G.H. Heilmeier et al., Proceeding of the I.E.E.E., Vol. 56, 1162-1171 (1968), and a guest-host cell proposed by G.H. Heilmeier, et al., Applied Physics Letters, Vol. 13, 91 (1968) and D.L. White et al., Journal of Applied Physics, Vol. 45, 4718 (1974).
The most current of these liquid crystal display cells is a twisted .nematic mode cell (TN cell), a kind of the field effect mode cells. As reported by G. Bauer, Mol. Cryst. Liq. Cryst., Vol. 63, 45 (1981), the TN cell requires that a product of an optical anisotropy (.DELTA.n) of a liquid crystal material filled in the cell and a thickness (d).mu.m of a liquid crystal layer in the cell should be set at a certain specific value in order to prevent generation of an interference fringe on the cell surface which would impair the cell appearance. In the liquid crystal cells for practical use. the above described product .DELTA.n.d is set at 0.5, 1.0, 1.6, or 2.2. In general, setting of the .DELTA.n.d value at 0.5 brings about satisfactory viewing-angle characteristics, and setting at 1.0, 1.6, or 2.2 brings about an improved contrast when seen from the front. Therefore, it is usual to set the .DELTA.n.d value at 0.5 in cases where a liquid crystal display cell is demanded to be easy to see from any direction, while, in cases where the cell is demanded to show a contrast from the front, the .DELTA.n.d value is set at 1.0, 1.6, or 2.2.
On the other hand, since the thickness of the liquid crystal layer in the currently employed cells is usually selected from a limited range of from 6 to 10 .mu.m, setting of the .DELTA.n.d at 0.5 naturally needs a liquid crystal material having a small .DELTA.n, while setting at 1.0 needs a liquid crystal material having a large .DELTA.n. Hence, whether a liquid crystal material should have a small .DELTA.n or a large An depends on the desired display characteristics of a liquid crystal display cell.
Most of the practical liquid crystal materials are generally prepared by mixing several kinds of compounds exhibiting a nematic phase in the vicinity of room temperature and compounds exhibiting a nematic phase at temperatures higher than room temperature. Since these mixed liquid crystals for practical use are required, in many cases, to exhibit a nematic phase at least over the entire temperature range of from -30.degree. C. to 65.degree. C., compounds having a nematic phase in the vicinity of room temperature or ih the temperature range higher than room temperature are needed.
Further, the mixed liquid crystals in the TN cell should have positive dielectric anisotropy (.DELTA..epsilon.), thus needing those nematic liquid crystal compounds whose value is positive.